1. Field of the Invention
The invention broadly concerns digital communication systems, such as spread spectrum systems. More specifically, reduction of the interference potential of relatively low-rate digital signals to other systems sharing the same frequencies is facilitated by spreading the low-rate digital data with a high-rate digital spreading sequence using pseudo-noise codes, for example, as used in Space Data communication systems.
2. Discussion of Related Art
In general, electronic communication systems transmit information by modulating an electronic signal representative of the data over a communication medium. A transmitting portion of such a communications system converts or modulates the digital information, i.e., baseband information, into a corresponding modulated signal for transmission through the communication medium. A receiving element receives the transmitted modulated signal and converts or demodulates the signal into the original digital information.
A variety of communication media may be used for transmitting the modulated signal. For example, the modulated signal may be transmitted through air or space utilizing radio frequency (RF), or using laser energy on optical fibers. RF transmissions are usually regulated by governmental authority and limited to a band of frequencies that is determined by regulatory and technology-based requirements. The range of frequency over which a communications system operates is often referred to as the bandwidth of the communication medium.
It is an ongoing problem in space communication systems to improve utilization of the available frequency bands. One way of increasing utilization is to multiplex a plurality of simultaneous users over the band with little or no mutual interference. Digital and analog signal processing techniques may facilitate improved bandwidth utilization.
One common digital signal processing technique is spread spectrum communication, which may be defined as any processing technique that digitally encodes the data for transmission in a format which disperses the data over the bandwidth. A spread spectrum transmission is such that multiple users may operate within the available bandwidth while incurring minimal interference from other users who are simultaneously using the same bandwidth. Each receiver decodes the received information in such a manner that each user's encoded data may be digitally extracted from the combined transmissions on the same frequency band or bandwidth.
Spread Spectrum techniques use a digital code to encode the data for transmission. In general, the user avails a transmitter-receiver pair including a transmitter element that spreads the data for transmission using a digital code. This code is temporally unique on the system bandwidth. A corresponding receiving element receives the spread information and despreads the transmission to recover the original transmitted data, i.e., the baseband information. In most instances, the receiving element must identify and use, for purposes of decoding the modulated transmission, the same code that the transmitting element has used to encode the baseband information in forming the modulated transmission. Other transmitter/receiver pairs may utilize available bandwidth on the same communication medium. The elements of the digital spreading codes are often referred to as “chips,” which are distinct from “bits” referring to units of user data.
A variety of digital spreading code types are generally known in the art. Collectively, all such codes may be referred to as pseudo-noise (“PN”) signals or pseudo-random noise (“PRN”) signals. The encoded information of one user is “spread” over the available bandwidth, such that the signal appears to be random noise having lower interference potential to any other users of the medium. Well known digital filtering techniques use the principle of processing gain to extract encoded information for the intended user without interference from the other users of the bandwidth.
Gold codes are one particular type of PN code. Gold codes are well understood by those skilled in the art, for example, as described in Optimal Binary Sequences for Spread Spectrum Multiplexing, IEEE Trans. Info. Theory, vol. IT-B, October, 1967, pp. 619-21 which is hereby incorporated by reference. In general, Gold codes provide a large number of codes and help minimize interference between users using the available bandwidth of the communication link. Gold codes are frequently utilized in GPS (global positioning system) satellite communications, in other satellite communication applications, and in such consumer communication applications as code-division multiple-access (CDMA), cellular radio and telephony.
Although PN codes advantageously provide one way to distinguish between multiplexed spread spectrum communications, it is problematic that a time delay factor may be associated with the use of PN codes. In certain spread spectrum communication systems, the receiver may first acquire the PN code signal by adapting to phase shifts in the communicated signal. Techniques for such code acquisition or bit synchronization using serial search methods are well known. Although the receiving element may know which PN code is being used to encode the data, the receiver may not be synchronized with the transmission of encoded units of information. In other words, the receiver needs to synchronize with individual codes that are located in a received stream of chips. Once the code is so acquired, the receiver may continue decoding the received information in synchrony; however, this synchronization process takes time. Code acquisition delays may prevent full utilization of the available bandwidth
More particularly, many CDMA communication systems use what is known as correlation receiver architecture to acquire the transmitted code sequences. This architecture acquires the code by receiving an appropriate number of chips, which the system presumes represent the utilized PN code. If the received chips appear to correlate to an expected encoded data bit value of “1” or “0” within a desired threshold of correlation, then the PN code has been acquired and decoding of data continues. If the presumed PN code does not adequately correlate to an expected data value, additional chips may be received and the sense testing for “1” or “0” repeats until acceptable correlation indicates that the PN code is acquired. Design and operation of such correlation receivers are generally known in the industry, for example, as described in E. Kaplan, Artech House, GPS receivers (1996)(see FIGS. 5.2, 5.3 and 5.13, Understanding GPS Principles and Applications).
Another general synchronization approach known in the art is to apply a matched filter that samples the incoming stream by shifting it through a shift register and comparing the shift register against an expected repeating sequence until it recognizes the expected sequence. This approach is not compatible with complex PN codes, such as Gold codes, because the repeating sequence may be extremely long and hence render a matched filter approach impractical.
Another approach suggested by Ward in Acquisition of Pseudonoise Signals by Sequential Estimation, (IEEE Trans. on Comm. Technology, Col. COM-13, No. 4, December 1065, pp. 475-483), is often referred to simply as sequential estimation. This approach differs from that of the correlation receiver architecture, which requires active closed-loop control of the rate and the relative time delay of a locally-generated code sequence that is used, as described above, to serially search for and achieve synchrony with the received chip sequence. In contrast, the sequential estimation receiver of Ward contains one code generator that is immediately in synchrony with the received chip sequence following receipt and logic-loading of the N-th chip. Therefore, the sequential estimation receiver advantageously does not have to search for synchronism as does a correlation receiver. Moreover, the correlation receiver must continually make active adjustments to its closed-loop to maintain synchrony with the received chip sequence. The code generator in the sequential estimation receiver of Ward is clocked by the receiver's timing recovery circuitry, which derives its timing from the received chip sequence so that changes in the rate of the received chip sequence, e.g., rate changes due to Doppler effects, are self-compensating. Though sequential estimation provides numerous benefits over correlation receivers, a particular problem with these known approaches arises in their application to Gold codes.
In general, a Gold code generator comprises two N-stage shift registers. Various ones of the stages (i.e., the “taps”) may be summed modulo-2 and the sum applied as feedback into the shift registers. The output of the two shift registers, which is the modulo-2 sum of the respective taps, may be summed modulo-2 to generate the Gold code output of the generator. By configuring the taps of the registers and by pre-loading the shift registers with particular values, the Gold codes generated by the generator may be varied. Since Gold codes represent the modulo-2 sum of two values, each received chip represents the modulo-2 sum of two bits in the paired shift registers that make up a Gold code generator. The techniques for sequential estimation as taught by Ward do not easily adapt to Gold codes, as used for spread spectrum communication systems. The presently known techniques would require significant additional processing of received chips to acquire desired code synchronization. Although Gold codes provide the benefit of improved spreading and separation of multiple users, and hence better bandwidth utilization, code acquisition of Gold codes from a received data stream is relatively more complex and time consuming. This necessitates the expenditure of extended time for achieving synchrony or lock. The time delay is undesirable in certain high speed applications where acquisition of the Gold or PN code must be attained more quickly.
It is evident from the above discussion that a need exists for improved methods and structures to provide rapid acquisition of pseudo-noise digital codes in spread spectrum digital communication systems. In addition, a simple structure for improved Gold code acquisition in spread spectrum digital communication systems is of particular utility.